workflow-fix: Step 5a spec-sync add/modify-only leaves stale main-deleted family twins — TG scan leg collects them and crashes the Step 10d gate

Goal

Close the Step 5a spec-freshness sync's stale-deletion blind spot: the family-atomic sync (.claude/skills/issue/steps/09-step-5.md L125-416, mirrored inline at 18-step-10d.md post-gate re-sync) uses git checkout origin/main -- <family paths>, which can only ADD/MODIFY — it never DELETES a worktree file that origin/main has removed or renamed away. A branch therefore keeps a stale main-deleted twin of a synced family file, and any glob-selected scan test collects the stale twin against the freshly-synced module.

Incident (task #2377, Step 10d lint gate run 1, 2026-08-19 04:08–05:15 PDT)

• main landed issue-2183 at 01:21 PDT: renamed the #2181 check check_no_repo_root_syspath_in_tests → check_no_repo_root_syspath (widened to scripts/) and removed tests/test_workflow_lint_no_repo_root_syspath_in_tests.py.
• The issue-2377 worktree's Step 5a syncs (03:59/04:04) brought the NEW scripts/workflow_lint.py but left the stale REMOVED test file in place (add/modify-only property).
• The Step 10d TG mapped leg's scan-test glob (test_workflow_lint*.py) collected the stale file → ImportError: cannot import name 'check_no_repo_root_syspath_in_tests' → collection error → pytest rc>1 → verdict crash → a ~67-min gate run wasted and one crash-case re-run consumed. Evidence: /tmp/issue-2377-tg-new.txt, task #2377 events (epm:progress v14).
• The property itself is documented (18-step-10d.md ~L2366: "can only add/modify, never delete") but only as a re-bind-stanza rationale — no arm PREVENTS the stale twin from breaking the TG leg.

Acceptance criteria

1. The Step 5a family-atomic sync (both copies: 09-step-5.md block + the 18-step-10d.md post-gate inline block, plus the Step 9c step-1a binding reference) also REMOVES worktree files under the synced family paths that are ABSENT on origin/main — bounded strictly to the family glob/path set, never payload files (a file present in the branch's own-diff vs merge-base is payload and is never removed), respecting the existing dirty-family skip (a dirty family syncs nothing and deletes nothing).
2. Alternatively (or additionally, as defense in depth): the gate's TG scan-test selection excludes test files that are absent on origin/main AND absent from the own-diff (stale-twin exclusion at collection time).
3. A regression test pinning the fire branch (stale main-deleted family test twin present → sync removes it / selection excludes it) and the keep branches (payload file kept; dirty-family untouched).
4. The no-lost-row agent-memory duty and the #1972 uncommitted-dirt arm survive unchanged.

Provenance

workflow_fix_target: .claude/skills/issue/steps/09-step-5.md Found by the /issue 2377 orchestrator during Step 10d (gate run 1 crash triage). Dedup key: (09-step-5.md, step5a-sync-stale-deletion-tg-scan-crash).